Popular Science Monthly/Volume 54/November 1898/The Torrents of Switzerland
|THE TORRENTS OF SWITZERLAND.|
By EDGAR R. DAWSON, M. E.
MARK TWAIN once said that he was in constant expectation in Switzerland of seeing a farmer fall out of his farm. The farmer has in many cases appreciated his hazardous position when harvesting his crops, and has put on crampoons to prevent a precipitous trip into the valley below. The crampoons prevent the farmer leaving his farm in such an undignified manner, but they do not prevent that same farm leaving its position on the mountain side. To show how, in many cases, the mountain sides are kept intact is the object of this paper. The old simile, "I am as sure of it as of the ground on which I stand," would be as much out of place in some parts of Switzerland as in those parts of the world where earthquakes are endemic. In fact, in these latter places, though the surface may receive a good shaking, it generally returns to somewhat the same neighborhood after its nervous peregrinations are over. Not so with the Swiss mountain side. When part of the mountain takes leave of the rest, it is forever.
Switzerland is often spoken of somewhat derisively as a garden, so perfectly have its pleasure grounds been laid out, and so completely comfortable does one find one's self in the midst of Nature's grandeurs. If its water courses had not been controlled and cared for as are those of a well-conducted park it would be chaos! The constant and vigilant struggle the Swiss have been forced to maintain against the liquid element is much to their credit, for they have generally been victorious. They have spent enormous sums of money in keeping their torrents and rivers within reasonable limits, and are even now, at times, forced to suppress new insurrections on the part of these irresponsible agents. The corrections of the water courses have been necessary for several reasons. In the first place, the erosions on the mountain sides result in deposits which present different inconveniences, of which I shall speak later. In the second place, the erosions are frequently the cause of landslides. The work of regulating the action of the water courses is now done according to accepted rules based on experience and on theories which have been confirmed by facts. Years ago, before the confederation took charge of this matter, it was done often in a haphazard, empirical fashion by the local authorities, with or without the aid of an engineer. But some great disasters in the canton of Grisons awakened the people to what might occur to many of them who had hitherto been more fortunate. At the end of September, 1868, both slopes of the Alps, and particularly the cantons of Valais and Grisons, were visited by floods of enormous magnitude. Such, was the devastation caused that an appeal was made to the generosity of the nation at large in behalf of the sufferers. This was responded to with such liberality that a large portion of the sum subscribed was put aside for the purpose of improving the water courses permanently. The fact that collective action was necessary in the attempts to control the turbulent streams became very apparent. This being the case, the state was called upon to take charge of this colossal enterprise. In July, 1871, by federal decree, the confederation declared that the correction and extinction of torrents was a matter of public utility, and worthy of the subsidies of the national Government. At the same time the relative burdens of the cantons and the confederation were settled. The importance of the improvement of the water courses and of the wooding of the regions where they rise was recognized in the Constitution of 1874. There the matter was definitely put under federal control and classed with the allied question of the conservation of the forests. The problem of keeping the waters under control in Switzerland ranges from the marshy lowlands to the summits of the passes. In spite of the varying conditions that this range entails, there are certain general principles that bear on all cases where the water is in movement. As the Swiss supplies his want of coal by harnessing his streams, so he makes the water do a large share of the work of correcting its erring ways and preparing itself to be harnessed. This he does by utilizing its power of carrying or depositing stones and soil, according as it is held within narrow banks or allowed to roam at will. As this power depends also on the steepness of the slope down which it runs, he uses this latter factor as well.
When he has got the water courses into what he considers good working condition and one that should be permanent, he tries to clinch matters. This is done by so combining the various conditions of cross-section, slope, and quality of soil that the action of the water is automatic—that is, it brings down no more earth and stones than it is capable of carrying below to safer places where the dangers of floods are small. Whenever the force with which the water moves along is stronger than the cohesion of the soil, erosion occurs. This erosion will continue, the channel of the stream becoming ever deeper, until a soil is encountered whose resistance is equal to the erosive action of the water. As the alluvion is carried on, the slope of the stream will become steeper and steeper the higher one goes. This circumstance would be of much greater importance if the gorges and gullies in which most of the streams run were not so well provided with rock. The power of the water to cause erosion is lessened in proportion to the amount of material it is carrying with it, which material is generally the product of previous erosions. Whenever the current needs all its strength to carry the material it has in suspension, together with the solid matter it is pushing along, it will have lost all its power to cause erosion. If the slope is decreased, or if the matter in suspension is increased in any manner, deposits will occur. These deposits render the slope less steep at that point but steeper below, so that the action of the water beyond will make itself felt, and by digging gradually up stream tend to restore the original slope.
In some cases there is very little erosion where the current is much stronger than the cohesive power of the soil, for the reason that the beds of the streams have been almost paved with stones that have been carried along by the propulsive action of the water.
This has in many cases produced an equilibrium between the resistance and the destroying power; in others the equilibrium has been brought about in a different way by the same natural agents. The current of a stream will very often go on causing erosion until arrested by some rocky obstacle that determines a waterfall. These falls cause breaks in the action of the water not only in stopping the erosive action in its upward march, but also in checking the velocity of the water. Then, as basins are often formed just above the falls and where the current is much less swift, matter in suspension is deposited, so that when the stream is swollen it has material to work upon, before starting to make the original slope steeper.
The subject divides itself broadly into two branches—the extinction of torrents and the correction of the water courses in valleys. In the extinction of the torrents various plans are resorted to, which give the current greater propulsive power, but at the same time they render necessary greater protection of the bed. This may be done by incasing it within walls of masonry (though other materials are used in some cases) or shortening meandering portions. In the latter plan the slope is increased, the fall being the same for a shorter distance. Currents that have been making dangerous deposits at certain points and causing dangerous erosions at others are treated by the above systems until the danger has disappeared or the money has given out. When the erosive action of the water is already too great, the material carried and then deposited by the stream is often made use of to consolidate banks that are threatened. Spurs are built out from such banks, and this tends to mend matters not only in forcing the water to take another channel, but also in causing deposits at the foot of the menaced bank.
The destructive effects of the current are arrested when the streams are not important by means of dams made of trunks of trees and wooden stakes, often strengthened roughly with stones. Where the streams are larger, and where the erosive action can not be modified by enlarging the channel, as is the case in many gorges, it is necessary to make an artificial bed for the stream and at times to supplement this by masonry dams. The dams are not permanent in their effects, for as soon as the basin immediately above the dam has been filled with deposits and the original slope of the stream has re-established itself, the products of the erosion pass over the dam. They prove, however, of great utility at times of large freshets by causing deposits which are subsequently carried down in much smaller quantities. This prevents the disasters that would be caused by sudden enormous deposits when the streams are swollen.
The prospect for many a narrow valley would be a sad one indeed if means had not been taken to prevent the lateral erosions so common with the mountain torrents and so productive of landslips. Where the stream has provided itself with a stony bed on which to roll, it often tries to do damage by leaving the stones and attacking the earthy banks. It is in these cases that the danger has proved so insidious; for until disasters actually occurred, in many instances, the undermining effect of the water was not suspected, being entirely hidden from view.
The most economical way of combating such cases is by making use of those points that by their formation arrest the erosive action. This is done by re-enforcing them in such manner as to produce a series of natural steps. The upward march of the excavating action of the water is stopped at least partially and temporarily at each step. Such a method is of great advantage when it is necessary to delay a more costly correction for financial reasons.
The experience gained since a connected system was begun in the management of the water courses has been of incalculable value, and many have been the landslips arrested and prevented by seeking their causes in the hidden erosive action of a small stream. In still another manner does the water threaten the mountain sides, and that is by permeating the soil (which is thus rendered much heavier), until it reaches a bed of rock or other layer that it can not penetrate. There it forms a layer of slippery mud on which the soil above slides bodily down. The method pursued in such cases is sometimes that of a ditch dug to the impermeable layer, sometimes that of a drain. In both cases there may or may not be small feeding ditches. Another plan which appears very contradictory of what has just been said is also employed. This consists in preventing, by means of horizontal trenches, the water from flowing off on the surface in the regions above the timber line. The water is forced to percolate through the soil and so reaches the wooded portions, where it would permeate anyhow, much more gradually than otherwise. The soil thus escapes the enormous increase of pressure due to the sudden absorption of great quantities of water, and consequently is less in danger of leaving its fastenings.
One of the most difficult of the problems that these torrents give rise to is that of their control where they suddenly enter a valley, and where the slope is consequently greatly decreased. The decrease of current entailed causes the deposit of stones and material at the mouth of the gorge, and the water then spreads itself over the valley. This occurs more or less regularly with certain torrents that are usually dry and where it is impracticable to prevent the erosions above. It then becomes necessary to build a stone canal from the mouth of the gorge to the principal water course of the valley. As this must be built on the alluvion (which presents the surface of a cone), it is often higher than the rest of the valley, and one may find other small canals for the draining of the valley passing under the larger one and meeting the principal stream below. A similar action to that of the torrent on entering the valley is that of a stream with a rapid current emptying into one whose current is slower. Here the deposits will at times force the smaller stream to seek another channel, and it frequently occurs that the correction moves the mouth of the stream a considerable distance.
The manner in which the streams in the valleys are made to aid in their own correction is most interesting. Whereas in the mountains it is usually desirable to decrease the erosive action of the water, in the valleys the contrary is the case, as the deposits in the lowlands are as dangerous to life and property as was their abstraction above. The great desideratum to be attained is to have the mountain streams arrive in the valleys in a purely liquid condition, and to give the valley streams the power to carry to the lakes any material they may be so unfortunate as to have taken in charge. To accomplish this latter purpose, the sinuosities of the streams are often reduced to straight lines, an increase of slope being thus secured. The new channels are made of a cross-section to enable the water to carry on its alluvion and silt. Where great freshets occur it is necessary to guarantee the artificial beds against the enormous increase of the water's destructive action.
The usual plan is that of having the cross-section of the stream with a deep depression in the center. This depression is of dimensions to insure a proper flow under ordinary conditions. When the stream becomes swollen it overflows the borders of this depression and spreads over a much larger area until the banks proper are encountered. This sudden increase of cross-section reduces the velocity of the water and consequently its destructive power. When the water of a stream is turned from its old channel into the new one that has been prepared for it, the operation is generally very gradually performed, so as to enable the water to fill up the old bed as much as possible by depositing its silt.
A plan pursued with much success is that of building out from the high bank of a stream (which it is desired to confine into a narrower channel) insubmersible spurs, stopping at the points where the new bank is to be situated. The water flowing in between these spurs deposits its dirt and gravel, and gradually builds up the new bank. In many cases the ends of the spurs are connected by low artificial banks of masonry over which the water flows. These banks retain and protect the deposits, and, when the latter have attained a sufficient depth, the artificial banks are raised to their permanent height. Still another method pursued with the same object in view is that of starting up stream and building the banks to their permanent height until a point is reached where it is desired to "fill."
Here the artificial banks are left temporarily very low. The water overflows them, and the reduction of its velocity entails the deposit of its silt. When this has continued as long as necessary or practicable, the walls are raised to their permanent height along the section and the same process is repeated below. This gradual process is also very advantageous from a financial point of view. When the engineer finally reaches the mouth of the stream at one of the lakes, we should expect to find his difficulties at an end, as the lakes are usually so deep that the alluvion makes little impression on them, and their areas are such that floods are not much to be feared. But he is confronted here with a new difficulty, that of anchoring or securing the foundation of his artificial river bank. The soil is generally alluvial over a large area, and is very damp. He generally has to terminate the masonry before he reaches the less stable alluvial soil and continue the structure by means of wooden material, which retains its position much better under such circumstances and is more cheaply replaced. It would be natural to imagine that man's control of the water problem stops at this point. But not so with the Swiss; he even controls its exit from some of the lakes. This is notably the case at Geneva, where by means of ingenious dams the lake is maintained at what is deemed a proper level. When it is remembered that this lake is fifty miles long by ten broad, an idea is gained of the amount of water controlled. Every few years the level is lowered for a given period, so that repairs may be made to the walls and structures along the shores.